During a ratio change of the gearbox, in the general case of a conventional motor vehicle, the driver first lifts his foot from the accelerator pedal which, as soon as the accelerator has reached the rest or zero torque demand position thereof, corresponding to complete raising of the pedal, brings about the immediate initiation of a transient progressive torque reduction phase comprising the closure of the air inlet valve(s) in the cylinders of the engine. The raising of the foot from the accelerator, referred to below as “raising the foot”, or “complete raising of the foot” detected by the engine control unit as a result of a position sensor which is associated with the accelerator pedal, thus represents for the pedal a zero torque request on the part of the driver.
The transient progressive torque reduction phase strategy involves in particular not immediately cutting off the injection at all the cylinders at the same time and thus prevents any excessively abrupt reaction of the vehicle for the passengers thereof, of the jerked slowing action type. In this manner, such a strategy involves in known manner, extending over a period of time which may be up to several hundreds of milliseconds, approximately from the raising of the foot:                a delay in injection cut-off, this delay being able to be in the form, for example, of a sequential cut-off of the injection of all the cylinders in a predetermined order,        a predetermined control of the progress to ignition,        a predetermined control of the reserve of combustive in the inlet conduit, in accordance with the transient progressive torque reduction control mode which has been implemented in the engine control unit.        
The control of the torque in the progressive torque reduction phase may bring about a surge of the engine speed during a gearbox ratio change, between the disengaged state and the engaged state, from the disengagement of the engine until the end of the transient progressive torque reduction phase. Such a surge of speed is unpleasant for the driver who may be momentarily surprised by the reaction of the engine.
When the vehicle is provided with an engagement sensor which is capable of transmitting to the engine control unit information relating to the engagement state, that is to say, disengaged or engaged, a surge of speed during a gearbox ratio change may be prevented as a result of this sensor by coupling it to the position sensor of the accelerator pedal which provides the information concerning the torque requested by the driver. This is because, with such a clutch sensor, the engine control unit may immediately and without delay cut off the injection of fuel as soon as the information relating to a disengagement has been received. In this instance, the immediate cut-off of injection takes priority over the progressive cut-off strategy of the injection applied when the foot is raised, which is therefore not activated since it has no use, the engine being disengaged.